Photochromism or phototropism, as the phenomenon has been variously termed, has been long known to science. Thus, certain naturally-occurring materials, e.g., titanium dioxide and Hackmanite, undergo a color change when exposed to light of one wavelength, followed by a reversion to their original color in the dark or on irradiation with light of a different wavelength. Also, families of organic compounds exhibiting photochromic behavior, such as the fulgides and spiropyrans, have been synthesized. One rather recent development has been the production of glass articles, most notably ophthalmic lenses, demonstrating photochromic properties.
U.S. Pat. No. 3,208,860 comprises the practical foundation for the development of photochromic glasses. That patent broadly discloses the incorporation of silver halide crystallites selected from the group of silver chloride, silver bromide, and silver iodide in silicate-based glass compositions to yield glass articles which darken (change color) when exposed to ultraviolet radiation and regain their original transmittance when removed from the ultraviolet radiation. The silver halide particles are postulated to react in some manner with the ultraviolet radiation but, being encased within a glass matrix, the reaction products cannot leave the vicinity of the crystallites such that, when the ultraviolet radiation is removed, the reaction products are available for recombination. Because of the essential hardness and rigidity of the glass matrix, the photochromic behavior of the glass articles is not subject to fatigue. That is, the degree of darkening and lightening is not altered by repeated exposures to and removals from ultraviolet radiation. That characteristic is of extrene importance in the fabrication of ophthalmic lenses since, to date, all organic materials used to fabricate photochromic ophthalmic or plano (non-prescription) lenses have met with failure, in that they have exhibited fatigue after a relatively few cycles of darkening and lightening, thereby losing their photochromic properties.
Whereas lenses prepared from organic plastics are much more prone to scratching in use than glass lenses and do not have the range of refractive indices that are available in glass lenses, they nevertheless have made inroads into the traditional glass markets of ophthalmic and plano eyeware, principally because of their light weight. This difference in density is of special interest to persons requiring prescriptions of very high power, which necessitates lenses of substantial thickness.
Since research to date has not discovered an organic compound evidencing photochromic behavior which does not fatigue, numerous schemes have been devised to in some manner combine inorganic photochromic materials with plastics, or to envelope organic and inorganic photochromic particles in protective coatings and then disperse those particles in a plastic matrix, or to hermetically seal a photochromic organic material within an organic envelope.
For example, U.S. Pat. Nos. 3,932,690, 4,168,339, and 4,300,821 are drawn to the fabrication of laminated articles wherein a layer of an inorganic material demonstrating photochromic behavior, either a sheet of glass or a layer of crystals, is buried within an organic plastic mass. Hence, U.S. Pat. No. 4,168,339 discloses the use of photochromic glass microsheet as the buried layer and U.S. Pat. No. 4,300,821 utilizes a mat of photochromic fiber glass for the same purpose. U.S. Pat. No. 4,168,339 also notes that, instead of burying the photochromic microsheet into a plastic mass, it may be bonded to the surface of a plastic mass which thereby protects the plastic from abrasion. U.S. Pat. No. 3,932,690 is directed to a three or four ply laminated article consisting of a glass or plastic substrate, a photochromic layer comprising silver and copper halide crystals, a plastic sheet, and, optionally, a glass or plastic sheet bonded to the plastic sheet.
U.S. Pat. No. 3,508,810 describes a laminated structure consisting of a pair of glass sheets sealed together with a resin containing an organic photochromic material dissolved therein.
U.S. Pat. Nos. 3,875,321, 3,950,591, and 4,035,527 are concerned with means for applying a glassy or crystalline coating of an inorganic photochromic material on the surface of an organic plastic substrate. Vapor deposition appeared to constitute the preferred process of application, although other method such as dipping, flame spraying, and sputtering were also observed as being operable.
U.S. Pat. Nos. 4,012,232, 4,046,586, 4,049,567, and 4,049,846 discuss several methods for incorporating organic and inorganic particles exhibiting photochromic behavior into plastic matrices. U.S. Pat. No. 4,012,232 discloses enveloping photochromic organic particles in protective inorganic coatings and then dispersing the coated particle in a plastic matrix. U.S. Pat. No., 4,046,586 describes enveloping photochromic inorganic halide crystals, preferably silver halide crystals, in inorganic non-oxide coatings, and thereafter incorporating the coated crystals into a plastic mass. U.S. Pat. No. 4,049,567 is concerned with growing silver halide crystals in an organic polymer environment, the polymer chosen having the capability of preventing the silver halide crystals from growing beyond a particle size of 1000 .ANG.; the environment also containing a sufficient amount of plasticizer to render the final plastic mass non-brittle. U.S. Pat. No. 4,049,846 is drawn to a method comprising the steps of forming a polymeric shape from a mixed polymer, swelling a surface layer on the polymeric shape with a polar solvent, absorbing silver and halide ions into the swelled surface layer, and then collapsing the surface layer by removing the solvent therefrom.
Each of the above disclosures is directed to the production of transparent products. However, those products have been plagued by various problems.
For example, other than fatigue experienced with the organic photochromic materials, the development of a strong, permanent bond between a glass sheet and a plastic substrate which will resist temperature and humidity changes in the service environment has proven difficult, and the elimination of haze due to light scattering from photochromic particles incorporated in the plastic mass has been perplexing. An extreme illustration of the latter problem is provided in U.S. Pat. No. 4,134,853. That patent describes the dispersion of very fine crystals, prepared by calcining a mixture of TiO.sub.2, FeO, and PbO which displays photochromic behavior, into a plastic mass. The plastic mass is shaped into a toy, such as a doll, which will appear to tan when exposed to sunlight. The final product is opaque even through the photochromic particles are reduced to sub-micron dimensions.
Nevertheless, in view of the extensive research disclosed in the above patent literature and the problems that have been witnessed with the articles resulting therefrom, it was determined that the ideal product would comprise a composite body consisting of photochromic glass particles dispersed within a plastic mass. Thus, a number of advantages in fabricating procedures and in the physical properties exhibited by the final body can be enjoyed in a composite product consisting of photochromic glass particles incorporated in a plastic mass, when compared with a glass-plastic laminate. For example, fabrication of a composite ophthalmic lens would make use of bulk glass (which is milled into a powder), rather than requiring the production of optical quality microsheet. That circumstance means that glass manufacture and subsequent heat treatment to develop photochromic properties therein are easier, and fewer forming constraints are imposed on the glass composition. That latter feature is of particular advantage in that essentially any inorganic photochromic glass composition would be operable. For example, microsheet exhibiting uniform photochromic properties is difficult to form from certain cadmium halide-containing glass compositions, such as are disclosed in U.S. Pat. Nos. 3,325,299 and 4,166,745, because the photochromic properties thereof are quite sensitive to cooling rates during forming. Uniformity, however, is improved in glass articles of substantial cross section and further homogenization can be achieved when the glass body is triturated. Furthermore, fabrication of a composite body is simpler than forming a laminated article. The principal advantage of such a composite body, when compared with a laminated structure, is the absence of delamination problems. Finally, the presence of the glass particles improves the resistance of the plastic mass to abrasion.